The present invention refers to a device for active sound control in a space with a sound field from at least one sound source according the preamble of claim 1.
A typical such device includes at least one pulse sensor, which senses a non-acoustic pulse signal of the sound source, for instance an engine of a vehicle. Consequently, the pulse sensor provides a pulse signal with a frequency which varies with a state of operation of the engine, i.e. the rotation speed of the engine. The pulse signal may thus include a fundamental frequency, for instance a number of ignition pulses per second and overtones to this frequency. The pulse signal is supplied to a detecting member, which is arranged to detect phase θ and frequency f of the pulse signal t(n). The device also includes a number of sound influencing members, normally loudspeakers, which are arranged to reduce the sound field in said space, and a number sound sensors, normally microphones, which each is arranged to sense the actual sound field in said space and provide an error signal.
The core of the device is a control unit which includes a signal supply device that receives the pulse signal and supplies a first signal, substantially consisting of sinusoidal components, to an adaptive filter of the control unit, which has a number of filter coefficients and generates a drive signal for each loudspeaker from the first signal. The signal supply device also supplies a set of second signals, substantially sinusoidal components, to a calculating member, which calculates the value of the filter coefficients by means of the second signals and the error signal and updates the adaptive filter by the calculated filter coefficients.
Furthermore, the control unit includes a clock which defines the clock pulses determining when the filter coefficients are to be updated. This updating is performed according to the formula:                     w        _            k        ⁢                   ⁢          (              n        +        1            )        =            ν      ⁢                           ⁢                        w          _                k            ⁢                           ⁢              (        n        )              -          μ      ⁢                           ⁢                        ∑                      m            =            1                    M                ⁢                                   ⁢                              e            m                    ⁢                                           ⁢                      (            n            )                    ⁢                                           ⁢                      x                          k              ⁢                                                           ⁢              m                        ′                    ⁢                                           ⁢                      (            n            )                              wherein x′km(n)=x(n)Skm(n)where    wk(n) is the filter coefficients for each k, i.e. each loudspeaker,    v is the so called leakage factor,    μ is the step length for the updating,    em(n) is the error signal, for each m, i.e. each microphone,    x(n) is the first signal,    Skm(n) is the so called impulse response, i.e. the impulse which each loudspeaker provides in each microphone,    Skm(n) is the estimated impulse response and    x′km(n) is the second signal or the filtrated first signal, i.e. the first signal has been filtrated by the estimated impulse response.
Such devices are well known today and described in public documents and patents, for instance in the book Active Noise Control Systems from 1996 by Sen M. Kuo and Dennis R. Morgan, and the U.S. Pat. No. 5,170,433.
The calculation of the second signal x′km(n), i.e. the filtering of the first signal by the impulse response, is a scalar multiplication, which is to be done for each combination of a microphone and a loudspeaker and for each clock pulse, and which requires a high calculating capacity of the device. This filtering may normally be performed by a part of the control software executed in real time.